This invention pertains generally to radar receivers and particularly to radar receivers used in semiactive missile guidance systems.
It is known in the art that coherence between a radar transmitter in an aircraft or on the ground and a radar receiver in a missile in flight may be achieved by monitoring the frequency at which the radar transmitter is operating to achieve, through the use of any known automatic frequency control (AFC) technique, the requisite control of the frequency of the first local oscillator in a missile in flight. When the requisite control is achieved, it is then possible to tune the first local oscillator so that intermediate frequency echo signals from a selected target fall within a fixed narrow band of frequencies regardless of any Doppler shift impressed on the radar echo signals from such target. As a result, then, proper performance of the guidance system may be obtained even when echo signals from unwanted targets or clutter are present, electronic countermeasures are attempted or signals from several radar transmitters are received.
One particular type of AFC known in the art is the one described in U.S. patent application, Ser. No. 579,171, filed May 20, 1975 by James Williamson, entitled "Adaptive Semiactive Missile Guidance System and Elements Therefor", and assigned to the same assignee as this application. Briefly, the just-referenced arrangement comprises a quadrature demodulator in a missile during flight responsive to intermediate frequency signals from a radar transmitter and to signals from a reference oscillator to produce difference signals which are in quadrature and have the same frequency. When such signals are passed through appropriate filters and are applied to a synchronous detector, a direct current signal whose amplitude and sense are indicative of any offset in frequency between the intermediate frequency signals from the radar transmitter and the signals from the reference oscillator. While the just briefly described arrangement, which in effect is a discriminator operative over a much wider band of difference signals than is possible with a conventional discriminator, is extremely useful in a semiactive missile guidance system, it is subject to the deficiency of any discriminator. That is to say, when the frequency of the intermediate frequency signal from the radar transmitter is equal to the frequency of the reference oscillator, the direct current signal out of the synchronous detector is at a null. Consequently, it is not possible to lock the frequency of the signals out of the reference oscillator to the intermediate frequency signals from the radar transmitter.